1. Field of the Invention
The present invention relates to a clock selecting technology for use with a synchronous network such as an optical communications network.
2. Description of the Related Art
In a particular network, clocks should be synchronized over the entire network. Such a network is referred to as a synchronous network.
As an example of such a synchronous network, there are transmission units which are interconnected with optical transmission lines corresponding to the SONET (Synchronous Optical Network) standard that is a United States synchronous network communication standard. Part of transmission units within the network are connected to exchanges directly or through repeating units.
In such a network environment, one clock generating unit supplies a high accuracy clock reference to transmission units in the network.
In recent years, synchronization messages that represent quality levels of various clock references supplied to transmission units, have been defined. Thus, each transmission unit is required to have a function for detecting, controlling, and sending such synchronization messages. The synchronization messages are sent as overhead information (control information) on the optical transmission lines or the like.
In this case, as a function for processing synchronization messages, each transmission unit is required to have a function for comparing synchronization messages received along with a plurality of clock references received from a plurality of transmission lines with which that transmission unit is terminated, so as to allow that transmission unit to synchronize with a clock reference having the highest quality level. In addition, each transmission unit is required to have a function for outputting a synchronization message corresponding to a quality level of a current clock reference as overhead information on a SONET optical transmission line with which the transmission unit is terminated. It is supposed that such functions will be widely used in the synchronous networks.
However, since a synchronization message is a new concept, in the conventional US synchronous network or the like, definitions with respect to transmission/reception of messages that represent quality levels of clock references such as synchronization messages and definitions with respect to switching of clock references according to reception of such messages, have not been made. In addition, definitions in the case that the quality of a clock reference of a received signal cannot be assured due to an occurrence of a fault of a system timing selecting unit, have not been made.
In the mode for processing a synchronization message, when a unit having the function for detecting the synchronization message from a received SONET signal or the like is non-installed, the synchronization message cannot be detected from the received signal. Thus, even if a remote station has sent a message that represents the prohibition of the use of the synchronization message as a clock reference to the local unit, the message cannot be detected. When the clock signal is used as a clock reference, a loop in which a clock with the same quality level is referenced in a particular region of the network take place. Thus, a clock with a higher quality level cannot be referenced. In other words, a timing loop may occur. This is a first problem of the conventional synchronous network.
Moreover, in the mode for processing a synchronization message, when a unit fault takes place in a unit that detects the synchronization message from a received SONET signal or the like, the synchronization message cannot be detected. Even if the synchronization message can be detected, its validity cannot be assured. Thus, even if a remote station has sent a message that represents the prohibition of the use of the synchronization message as a clock reference to the local unit, the synchronization message cannot be detected. When the clock signal is used as a clock reference, a timing loop may take place. This is a second problem of the conventional synchronous network and the like. A unit fault means a state in which a unit does not operate. Examples of a unit fault are the states in which an oscillator of a unit stops outputting a signal, an output signal stops, and an LSI becomes defective.
When a unit that does not have the function for detecting a synchronization message is installed instead of a unit that has the function for detecting a synchronization message (namely, an improper unit is installed), the synchronization message cannot be detected. Thus, even if a remote station has sent a message that represents the prohibition of the use of the synchronization message as a clock reference to the local unit, the synchronization message cannot be detected from the received signal. When the clock signal is used as a clock reference, a timing loop may take place. This is a third problem of the conventional synchronous network.
In the mode for processing a synchronization message, a unit that has a function for detecting a synchronization message from the SONET signal or the like, may be in a so-called out-of-service state (OOS state). The OOS state of the unit is treated as a non-installed state of the unit. At this point, the unit does not detect various alarms. Thus, when a unit fault takes place in the OOS state of the unit, a unit fault alarm is not sent. In addition, the validity of a detected message is not assured. Thus, when such a message is used, a malfunction such as a timing loop may take place. This is a fourth problem of the conventional synchronous network.
In the mode for processing a synchronization message, when a unit that has a function for receiving a SONET signal or the like is non-installed, the signal cannot be received. Thus, even if a remote station has sent a message that represents the prohibition of the use of the corresponding clock signal as a clock reference to the local unit, the message cannot be detected. When the clock signal is used as a clock reference, a timing loop may take place. This is a fifth problem of the conventional synchronous network.
In the mode for processing a synchronization message, when a unit fault takes place in a unit that has a function for receiving a SONET signal or the like, overhead data that contain a synchronization message cannot be extracted from the received signal. Even if the overhead data can be extracted, the validity of the message detected by a synchronization message detecting unit corresponding to the extracted overhead data is not assured. Thus, even if a remote station has sent a message that represents the prohibition of the use of the signal as a clock reference, the message cannot be detected. When the signal is used as a clock reference, a timing loop may take place. This is a sixth problem of the conventional synchronous network.
When a unit that does not have a function for receiving a SONET signal or the like has been improperly installed instead of a unit that has a function for receiving a SONET signal or the like, (for example, a unit that has a function for receiving an STS-1 (Synchronous Transport Signal 1: 51.84 Mbps) has been installed instead of a unit that has a function for receiving an OC-3 (Optical Carrier 3: 155.52 Mbps)), even if the improperly installed unit receives a correct signal and can detect a synchronization message, the validity of the synchronization message detected from the received signal of an incorrectly structured network is not assured. Thus, even if the remote station has sent a message that represents the prohibition of the use of the signal as a clock reference, the message cannot be detected. Thus, when the signal is used as a clock reference, a timing loop may take place. This is a seventh problem of the conventional synchronous network.
In the mode for processing a synchronization message, a unit that has a function for receiving a SONET signal or the like may be in the OOS state. At this point, the unit does not detect various alarms. Thus, in the OOS state, when a unit fault takes place, a unit fault alarm is not sent. In addition, the validity of the detected overhead data is not assured. Thus, the validity of a message detected from the overhead data is not assured. Consequently, when such a message is directly used, a malfunction such as a timing loop may take place. This is an eighth problem of the conventional synchronous network.
In the mode for processing a synchronization message, a line through which a SONET signal or the like is received may be in the OOS state. At this point, various alarms are not detected from the line. Thus, when a line fault takes place in the OOS state, a line fault alarm is not sent. In addition, the validity of overhead data received from the line is not assured. Thus, the validity of a message detected from the overhead data is not assured. Consequently, when such a message is directly used, a malfunction such as a timing loop may take place. This is a ninth problem of the conventional synchronous network.
When the accuracy represented by a synchronization message detected from a received signal (SONET signal or the like) is lower than the accuracy of an internal clock of the transmission unit, normally, a clock reference detected from the received signal (SONET signal or the like) is not selected. However, if such a clock reference is mistakenly selected, even if there is a clock reference with a high quality level (namely, the internal clock in this case), the quality of the entire synchronous network may deteriorate. This is a tenth problem of the conventional synchronous network.